1. Field of the Invention
The present invention relates to a refrigerant cycle system having a gas-injection structure for injecting gas refrigerant from a gas-liquid separator to a compressor. The refrigerant cycle is suitable for an air conditioning apparatus for a vehicle such as a hybrid vehicle and an electrical vehicle.
2. Description of Related Art
As shown in FIG. 4, a conventional refrigerant cycle includes a compressor 122 having an outlet port 122a, an inlet port 122b and a gas-injection port 122c. The compressor 122 is a scroll type in which the gas-injection port 122c introduces middle-pressure gas refrigerant into the compressor 122 while the compressor 122 compresses refrigerant, for example.
When an air conditioning apparatus for a vehicle is in a heating mode, high-pressure high-temperature gas refrigerant compressed by the compressor 122 is introduced into a first heat exchanger 121 disposed in an air duct 102 provided inside a passenger compartment of the vehicle. The first heat exchanger 121 performs heat-exchange between air blown by a blower 106 and the high-pressure high-temperature gas refrigerant flowing through the first heat exchanger 121. Thus, during the heating mode, the refrigerant is condensed and liquefied in the first heat exchanger 121, and air is heated by absorbing heat from the refrigerant to be warm air.
The liquid refrigerant condensed by the first heat exchanger 121 is decompressed (press-reduced) in a high-pressure side expansion valve 126 (i.e., first decompressing means) until it becomes in middle-pressure gas-liquid refrigerant. The middle-pressure gas-liquid refrigerant is introduced into a gas-liquid separator 127 to be separated into gas refrigerant and liquid refrigerant. The gas refrigerant from the gas-liquid separator 127 flows through a gas-injection passage 129, and is introduced into an compression-intermediate portion of the compressor 122 through the gas-injection port 122c.
The liquid refrigerant from the gas-liquid separator 127 is decompressed by a low-pressure side expansion valve 128 (i.e., second decompressing means) until a predetermined low pressure to be gas-liquid two-phase refrigerant. The gas-liquid two-phase refrigerant from the expansion valve 128 is evaporated in a second heat exchanger 124 disposed outside the passenger compartment, by absorbing heat from outside air (i.e., air outside the passenger compartment). This gas refrigerant from the second heat exchanger 124 is introduced into the compressor 122 through the inlet port 122b. The expansion valve 128 is a thermal expansion valve, which detects a super-heating degree of the refrigerant to be sucked into the inlet port 122b of the compressor 122 and controls a flow rate of the refrigerant flowing into the second heat exchanger 124 according to the detected super-heating degree. Thus, the expansion valve 128 prevents liquid refrigerant which cannot be evaporated in the second heat exchanger 124 from being introduced into the inlet port 122b of the compressor 122. In the above-described refrigerant cycle with the gas-injection function, heating capacity of the air-conditioning apparatus is improved as compared with that of an air conditioning apparatus having a refrigerant cycle without the gas-injection function.
Lubricating oil is used for the compressor 122 so that a sliding portion of the compressor 122 moves smoothly. The lubricating oil is dissolved in liquid refrigerant and circulates through the refrigerant cycle along with liquid refrigerant. In the gas refrigerant area of the refrigerant cycle, the lubricating oil circulates through the refrigerant cycle while being washed away by gas refrigerant along an inner wall of a refrigerant pipe. That is, the lubricating oil dissolved in liquid refrigerant in the gas-liquid separator 127 passes through the low-pressure side expansion valve 128 and flows into the second heat exchanger 124 along with the refrigerant flow. The lubricating oil from the second heat exchanger 124 is returned to the compressor 122.
However, when the air conditioning apparatus is used at an extraordinary low temperature such as -20.degree. C., the evaporating temperature of refrigerant becomes lower -30.degree. C. in the second heat exchanger 124 due to decrease of the opening degree of the low-pressure side expansion valve 128. As a result, viscosity of the lubricating oil may become extremely high, resulting in adhesion of the lubricating oil to the inner wall of the refrigerant pipe. Therefore, the lubricating oil can not flow by the gas-refrigerant flow. That is, at an extremely low temperature as described above, the lubricating oil hardly flows from the second heat exchanger 124 to the compressor 22; and therefore, durability of the compressor 122 is decreased. Especially, when the compressor 122 is a scroll-type, abrasion of a sliding surface in the vicinity of the outlet port 122a of the compressor 122 may cause a big problem.
JP-A-3-260556 discloses a refrigerant cycle with a gas-injection function, which introduces not only gas refrigerant in a gas-liquid separator but also liquid refrigerant in the gas-liquid separator into a compressor through a gas-injection passage. In the refrigerant cycle, the gas-liquid separator is provided with a gas-suction pipe connected to the gas-injection passage of the compressor. In the gas-liquid separator, the gas-suction pipe is immersed in a liquid-refrigerant area, while an opening end of the gas-suction pipe is positioned in a gas-refrigerant area. Further, the immersed part of the gas-suction pipe has a suction hole for sucking the liquid refrigerant. Therefore, both gas and liquid refrigerant in the gas-liquid separator can be introduced into the compressor 22 through the gas-injection passage.
In the above-mentioned refrigerant cycle, liquid refrigerant is sucked into the gas-suction pipe through the suction hole by suction power of gas refrigerant flowing through the gas-suction pipe. Therefore, quantity of liquid refrigerant returning to a compressor through the gas-injection passage is determined according to a flow rate of gas refrigerant flowing through the gas-suction pipe and an opening area of the suction hole. However, the suction hole having a small diameter may be blocked due to dirt and dust contained in the refrigerant, and it is difficult to suck liquid refrigerant to be returned to the compressor during a long period. On the other hand, if the suction hole is made larger, too much liquid refrigerant returns to the compressor through the gas-injection passage, resulting in deterioration of efficiency of the refrigerant cycle. Further, in this case, the operation of the compressor may be affected due to compression of liquid refrigerant by the compressor.